Increasingly, drilling contractors are using top drives instead of Kellies or Kelly bushings. A top drive is a drilling tool that hangs from the traveling block, and has one or more motors to power a drive shaft to which crewmembers attach the drill string. Because the top drive's motor can rotate the drill string, no Kelly or Kelly bushing is required. The top drive also incorporates a spinning capability and a torque wrench. In addition the top drive has elevators on links. The benefits of top drives may include the ability to work in 90 feet increments rather than the 30 feet increments to which a Kelly is typically limited. That is, a joint of tubular is typically 30 feet long. Thus, a top drive allows an operator to work with 3 joints of tubular per increment of a given operation. For example, top drives allow operators to assemble three-joint (90 feet) stands of tubular off the critical path to save time. Similarly, in some instances, such as, for example, applications involving horizontal or highly deviated well bores, it may be desirable to remove tubular from a well bore by a process known as back reaming. A top drive allows operators to back ream tubular from a well bore in three-joint stands of tubular, which may then be racked intact.
On a drilling rig, the critical path includes all tasks and equipment required to continue drilling without interruption. When a task or equipment on the critical path is delayed, the entire drilling operation is delayed. Thus, because mechanical devices require some amount of repair and/or maintenance, many drilling rig critical path components are maintained in redundant quantities to decrease downtime caused by inevitable repair and maintenance. Conventionally, top drives have been an exception to this principle of redundancy. Because top drives are generally on the critical path, top drives create the potential for single point failure—that is, if the top drive goes down, the entire drilling operation stalls, rendering the entire rig nonoperational until the top drive can be brought back online. Generally, diagnostics occur in the critical path before any repairs can be done, causing additional delay in the operation before repair even begins. Likewise, maintenance operations can fall within the critical path, creating downtime.